1. Field of the Invention
The present invention relates to an automobile telephone system or portable telephone system (cellular system) based on direct spreading code division multiple access (DS-CDMA), and more particularly to a circuit for detecting a spreading code used by a base station in a cell where a mobile terminal of a cellular system is present.
2. Description of the Related Art
One conventional CDMA cellular system is a North American standard cellular system known as IS95. According to IS95, all base stations use a relatively long spreading code which is of about 26.6 ms (=215 chip period). The phase or starting time of the spreading code is shifted different intervals for the respective base stations to allow a cellular terminal to distinguish the base stations from each other.
In the case where the phase or starting time of the spreading code is shifted different intervals for the respective base stations, since the cellular terminal only needs to search for the single spreading code when it is switched on, initial synchronization only can be performed using a simple sliding correlator. However, because the shifted code is distinguished only with the phase or starting time, if the base stations are shifted out of synchronism for transmission timing, then it is impossible to differentiate the spreading code phases for the respective base stations.
For the above reason, it is necessary for all the base stations to be synchronized with an accuracy of several chips, e.g., several microseconds for IS95. According to IS95, the base stations are maintained in synchronism with each other by the GPS (Global Positioning System).
Another conventional CDMA cellular system is known as W-CDMA (wide-band code division multiple access) that has not yet been put to practical use but for which standardization efforts are presently being made by Association of Radio Industries and Businesses (ARIB) in Japan.
According to W-CDMA, accurate synchronization of a plurality of base stations is achieved not by shifting the phase or starting time of a spreading code between the base stations, but by using different spreading codes, e.g., Gold codes, for the respective base stations. Since the spreading codes used between the base stations are different from each other, the spreading codes of the base stations can be distinguished from each other even when the base stations are not kept in accurate synchronism with each other.
In order to identify a base station, however, a cellular terminal has to search all possible spreading codes. Therefore, a large-scale piece of hardware and a long period of time are required to perform a process of searching for a cell where the base station is located.
For reducing the period of time required for a cell search, there has been proposed a two- or three-stage search scheme for use with W-CDMA.
According to the three-stage search scheme, spreading codes (long codes) used by base stations are divided into a plurality of groups, and each of the base stations periodically transmits, in addition to an ordinary transmission code, a first short code (1), common to all the base stations, indicative of the starting time of the transmission of the spreading code, and a second short code (GID) indicative of the group number of the used long code.
The three-stage search process comprises first, second, and third stages.
In the first stage, the cellular terminal searches for the first short code to detect the starting time of the transmission of the long code.
In the second stage, the cellular terminal detects the second short code (GID) to identify the group number of the used long code. Since the group number is identified, the number of long codes which have to be searched can be reduced to a fraction thereof inversely proportional to the number of groups.
In the third stage, the cellular terminal uses all possible long codes to de-spread the received signal to decide whether a long code that can properly be received is an actually used long code or not.
The above conventional CDMA cellular systems suffer the following disadvantages:
(1) Because the shifted code is distinguished only with the phase or starting time, if the base stations are shifted out of synchronism for transmission timing, then it is impossible to differentiate the spreading code phases for the respective base stations. Thus, as with the IS95 system, if all the base stations use one spreading code shifted in phase, then the CDMA cellular system needs a synchronizing signal to keep all the base stations in accurate synchronism with each other for transmission timing. The CDMA cellular system could not be constructed in the absence of an appropriate external synchronizing signal such as a synchronizing signal from the GPS, for example.
Furthermore, the GPS is basically a U.S. military system and may possibly be put out of service for security reasons. If the GPS becomes inoperative unexpectedly, then the CDMA cellular system is abruptly shut off. Another problem is that areas where GPS antennas cannot be installed, e.g., indoor areas, underground areas, etc. cannot be used as service areas of the CDMA cellular system.
(2) In the CDMA cellular system, one frequency is shared by a plurality of channels, and hence it is necessary to minimize interchannel interference. To meet such a requirement, signals are transmitted with a minimum required level of transmission power in each channel. In order to identify a spreading code that is being used, the spreading code needs to be actually de-de-spread to decide whether the spreading code is being used or not. Since it is necessary to de-spread all possible spreading codes in an exhaustive fashion, the process of detecting whether there is a signal or not is complex.
It is therefore an object of the present invention to provide a CDMA cellular system which does not require accurate synchronization between all base stations and is capable of distinguishing the base stations from each other without the need for a complex process, and a method of detecting a spreading code in such a CDMA cellular system.
According to the present invention, when two element codes are multiplied with shifted timing to generate a spreading code, different codes that do not agree with each other when cyclically shifted are generated for the respective base stations. Spreading codes of the base stations are distinguished even when the base stations are not kept in synchronism with each other.
In a CDMA cellular system which uses such spreading codes, when frame synchronization is achieved using a perch channel spread by a spreading code that is common to all the base stations, a signal transmitted from one of the base stations is de-spread on the basis of a first element code which is of the same timing (phase) with respect to a frame in all the base stations. Since the de-spread signal is a signal produced by shifting a second element code with timing (phase) which differs from base station to base station, a spreading code that is being used can be identified by a correlated value calculating unit.